I like to yell when DJI does crap.  But, hats off!  

According to the new 350RTK documentation
1/the new batteries of the 350 are compatible with the 300.

2/The batteries of the 300 are compatible with the 350.

3/The RC plus is compatible with 300 - 350 - 30.

4/The unlocked arm warning device  it's a good job.  

For the rest we will see with use.  A competent guy had to take charge of the situation.  They understood that we don't care about design.  What we want is gear that holds up, without having to buy everything again with each new generation.

I agree, it's almost like a third party was in charge of designing the M350. DJI doesn't offer this kind of backwards compatibility with new product releases. I can't think of a drone ever receiving an Ocusync version upgrade. The M300 just magically jumped 2 generations with just a firmware update. I'm curious to see if the M300 with an RC Plus gets the same range as an M350.